This invention relates to machines which are guided in their operation by laser responsive guidance/control systems and, more particularly, to an apparatus and method for accurately controlling laser guided machines such as concrete screeding apparatus, earth movers and graders, floor cutting saws, floor finishing machines and the like.
Recent years have witnessed the increasing use of laser beam guidance/control systems on various types of machines. Typically, a rotating laser beacon or projector is positioned externally of a machine such as a grader or concrete screed such that the rotating beacon of the projector creates a precisely located plane of laser light in the area surrounding the rotating beacon. A machine to be guided by the laser plane includes one or more laser beam receivers which generate electrical signals when the machine or machine element on which the receiver is mounted is off-center from the laser beam, i.e., high, low, left or right of the laser plane. In one presently existing system, electrical signals from the laser beam receiver are processed through an electrical control circuit which is connected to appropriate valving to operate fluid cylinders which move the machine element in a defined direction relative to the sensed laser beam.
A principal reason for use of laser guidance on machines is to increase the accuracy of the resultant work product such as the area being graded or the concrete being finished. However, experience with existing systems has shown that the accuracy of the system depends on the accuracy of the laser beam receiver and the inherent "dead band" which exists when the laser receiver is centered "on target" with the laser beam. For example, in the existing system mentioned above, because of the width of the laser reference beam, the "on target" or "dead band" range is one-half to three-quarters of an inch. This means that the greatest accuracy obtainable with the system is plus or minus one-quarter to three-eighths of an inch. In many situations, such tolerances are simply too great as in, for example, finishing concrete where accuracy in height of the concrete of one-sixteenth to one-eighth of an inch is required.
In an effort to overcome the above laser guidance tolerance and accuracy deficiencies, especially in laser guided concrete screeds or finishing machines, previous methods have included adjusting the physical position of the machine with respect to the laser plane in order to reduce the dimension of the "on target" range or dead band. For example, with the laser guided screeding machine shown in FIG. 3 herein, and disclosed in co-pending U.S. patent application Ser. No. 07/291,678, filed Dec. 29, 1988 entitled IMPROVED SCREEDING APPARATUS AND METHOD, the screeding machine is positioned with its screed support boom at an approximate 2% slope such that its free end is lower than the supported base end. As the screed assembly is operated and moved toward the base of the machine to finish the poured concrete, it steadily rises due to the sloped position of the boom. As a result, if the screed assembly begins "on target" such that the laser receiver is centered with respect to the laser plane, it will rise to the "high" range above the "on target" range in a short distance. The control valving will then lower it back to the "on target" position. This pattern will repeat and provide a saw tooth pattern with an approximate one-eighth inch amplitude without traversing the wider dead band normally associated with laser receivers and without ever operating in the low range. This setup method is referred to as working off the "top edge of the laser beam".
However, working off the top edge of the laser beam requires precise positioning of the machine each time the machine is set up to finish an area of concrete. In a typical day, such machine may be moved and positioned as many as one hundred times. The additional positioning of the boom at a 2% slope adds a significant time factor to the finishing operation thereby increasing the cost of finishing the concrete. In certain situations, positioning of the machine at the 2% slope is also quite difficult. Further, in some situations, it is not possible to create the 2% slope such as when the screeding machine is being moved while finishing concrete behind the machine. In other machines, such as earth movers and graders not having cantilevered booms like the screed, there is no appropriate method for physically creating a slope on which the machine can operate. Hence, use of an artificial physical setup as a means for overcoming the above tolerance and accuracy problems is impossible.
Accordingly, a need was apparent for an apparatus and method suitable for controlling laser guided machines which would provide and maintain greater accuracy in the work product resulting from such machines and allow use on a wider range of machines including those being moved while operating.